Bounce compensator for free piston engines



March 13, 1962 A. J. EHRAT ETAL BOUNCE COMPENSATOR FOR FREE PISTONENGINES Filed Deo. 25, 1958 United States Patent() 3,024,591 BOUNCECOMPENSATOR FOR FREE PISTON ENGINES Alfred J. Ehrat, Norwalk, and JohnB. Williams, Greenwich, Conn., assignors to American Machine & FoundryCompany, a corporation of New Jersey Filed Dec. 23, 1958, Ser. No.182,519 4 Claims. (Cl. 60-19) The invention relates to free pistonengines and more particularly rto free piston engines in which the poweroutput of the engine is delivered in the form of a hydraulic tluid underpressure to one or more hydraulic motors.

In engine systems of the above type, hydraulic pump plungers arecustomarily operatively connected to the power pistons and serve toimpel fluid under high pressure to one or more hydraulic motors.

For the purpose, primarily, of smoothing out the pressure lluctuationsdue to the intermittent action of the reciprocating pump plungors, ahigh pressure accumulator is interposed between .the pump and the motoror motors and a low pressure accumulator is interposed in the returnline between the motor or motors and the pump plungers.

In tree piston engines, means are provided for returning the pistons.This means, as a rule, consists of a low pressure accumulator whosefluid is conveyed during the compression strokes to the pump pistons forreturning the pump and the power pistons to their inner position. -Thelow pressure accumulator thus provides the so-called' bounce pressurefor the hydraulic pump plungers and lfor this purpose a closed hydraulicsystem has been used in which the pressure in the low pressureaccumulator is utilized to provide the bounce force, the pressure in thelow pressure accumulator being regulated to maintain a value suicicntfor producing the compression stroke.

The maintenance of the required bounce pressure does not present anyserious ditiiculty when the engine operates always at constant load andwhen the pressures in the system are maintained at generally constantvalues. These conditions are readily attained when the engine iscontrolled in such a way that it starts and stops in response toaccumulator pressure, thus `maintaining nearly uniform pressureregardless of motor output fluctuation.

In orderto utilize constant hydraulic pressure conditions, however, itis necessary to control motor output by changing the volumetricdisplacement of the motors. Formany installations this presents greatdiculties.

Lf it were attempted to use constant displacement motors, or motorshaving inadequate displacement variability, in a system having varyingvaccumulator pressure,

. the bounce pressure supplied by the low pressure accumulator would befound to be unsuitable 'for providing the bounce forec requirements.Thus, for example, if the `motor torque and the power impulse laresimultaneously increased to'mect av demand for increased power output,the pressure in the low pressure accumulator will decrease on account ofthe greater pressure drop across the motors. Consequently, the availablebounce pressure is reduced, inadequate compression is attained andypower output is seriously impaired.

The present invention overcomes the inadequacy of hydraulic bounce byintroducing a resilient element, such as a auxiliary accumulator, in thehydraulic line between the pump plungers and the check valves whichcontrol the flow to and from the high and low pressure accumulators.4The auxiliary accumulator may take any one of several forms such asarelatively small hydropneumatic 3,024,591 Patented Mar. 13, 1962 ICCpressure uctuation incident to the reciprocation of the plungers. Ofthese instrumentalitcs the last named, which depends for its functioningonly on the elastic compressibility of the hydraulic fluid and, to alesser degree,

the elastic expansion of the container, is preferred on account ofsimplicity and freedom from the fatigue failure properties of rapidlyoscillated membranes or springs.

1 Since the volume fluctuation required is relatively small,

auxiliary accumulator, a spring-loaded piston in'a cylthis safe anduncomplicated expedient may be applied without objectionable bulk andweight.

Since the maximum pressure in the compensating vessel, or the auxiliaryaccumulator, increases with increased motor torque and piston impulse,it provides a bounce impulse which varies oppositely to the adverseVariation of the pressure in the low pressure accumulator. Thus, withsuitable proportioning of pump displacement and the auxiliaryaccumulator volume, a corrective impulse may be obtained on every strokecapable of substantially neutralizing the adverse response to loadiluctuation of the lowprcssure accumulator bounce action, Operation ofthe free piston engine with a closed hydraulic transmission system isthus made practicable with load uctuations over the widest range thatmay be required. In order .to obtain optimum compensation, the volumesand the elastic properties of the two accumulators should be l properlyproportioned to provide the desired compensation of the bounce pressure.

It is, therefore, an object of the invention to provide a free pistonengine with hydraulic power transmission capable of satisfactoryoperation with variable load.

Another object of the invention is to provide a hydraulic .transmissionsystem for a free piston engine which accommodates itself to varyingload without recourse to complicated regulatory instrumentalities.

Still another object is the provision of a resilient compensatorv ofsimplicity and durability for use in the hydraulic transmission of afree piston engine.

It is also an object of the invention to provide a free piston enginehaving two high pressure accumulators and a low pressure accumulator,the power stroke of the engine subjecting the working tluid in the twohigh pressure accumulators to a high pressure, one of the high pressureaccumulators supplying the hydraulic motive power to a hydraulic motorfor producing useful work, while the second high pressure accumulatorand the low pressure accumulator, in combination, supply anapproximately constant bounce pressure for producing a compressionstroke in the engine, this stroke being substantially independent of thevariable load conditions imposed upon lthe hydraulic motor.

It is an additional object of this invention to provide a free pistonengine utilizing a high bounce pressure accumulator and a low bouncepressure accumulator for producing a compression stroke in the enginewhich is reasonably independent of the variation in an external loadconnected to the engine.

These and other objects will become more apparent from the followingdescription and from the accompanying drawing illustrating a simpleembodiment thereof, in y which:

'Ihe only FIGURE illustrates a diagrammatic longitudinal cross-sectionof the engine.

Referring'to the only FIGURE in the drawing, it illustrates, indiagrammatic longitudinal cross-section, a free piston engine withhydraulic transmission arranged in accordance with one embodiment of theinvention.

The engine, generally designated by 10, is of the twocycle diesel type.vThe power pistons 22 are arranged to slide in the cylinder 20 in whichports 34 and 36 are provided for the admission of air and for thedischarge of exhaust gas, respectively. The scavenging pistons 48,sliding in the enlarged portions of the cylinder 20, in

conjunction with the check valves 45 and 46 supply air to the inletports 34 through ducts 11 and 12. lntegral with the pistons 22 and 48are the pump plungers 32 slidable in cylinders 28.

The fluid-carrying duct 58 connects the two pump cylinders 28. Duct 58is connected through check valve 60 and duct 61, accumulator 50 and duct65 to a hydraulic motor 78 of any suitable known construction.

Duct 64 connects motor 78 to a low pressure accumulator 98 and to duct62. Duct 62 is connected through a check valve 76 to duct 58.

A resilient compensator 63 is connected to duct 58. Compensator, or theauxiliary accumulator, 63 may take the form of a simple receptacle, ofspherical or other convenient form, lled with the working liquid.

A tube 38 is connected to a fuel injection device (not shown) whichserves to spray fuel into the combustion chamber 13 when the pistons 22are near the inner ends of their strokes, in accordance with the usualmanner of diesel engine operation. A hydraulic motor 78 is connected toa shaft 79 which drives a vehicle, machine or other external load 99.Motor 78 is connected through a duct 65 to the high pressure accumulator50 on the high pressure side and to the low pressure accumulator on thelow side through a duct 64. Motor 78 may be a constant displacementmotor which is never shut off from lines 65 and 64. Shaft 79, therefore,may rotate continuously at a speed corresponding to the operatingfrequency of the free piston engine 10.

In the position shown, pistons 22 have reached the outer limits of theirtravel at the end of the power stroke and have started back on theircompression strokes under the influence of the bounce pressure exertedon the pump plungers 32 by the working uid contained in the auxiliaryaccumulator 63 and the low pressure accumulator 98.

Pressure in the passage 58 and accumulator 63 has been raised to that ofthe high pressure accumulator 50 by the outward strokes of the plungers32 and has begun to diminish because of the inward travel of the pistonsand cylinders 28. Since, during the power stroke, the accumulators 50and 63 are connected in parallel, with the check valve 60 wide open, theworking Huid attains the same equal maximum pressure in bothaccumulators at the end of the power stroke. When the outward travel ofthe pistons ceases at the end of the power stroke, check valve 60 closesimmediately upon the beginning of the compression stroke and thecompression stroke takes place because of the pressure exerted by theliquid tirst supplied by the auxiliary accumulator 63 and then by thelow pressure accumulator 98, which completes the compression stroke.

The high pressure remaining in passage 58 due to the resilient action ofaccumulator 63 produces a rapid acceleration of pistons .22, thepressure in compensator 63 falling olf as they move inward. When pistons22 have moved through a portion of their inward strokes, as determinedby the elastic capacity of accumulator 63 and the area of the plungers32, the pressure in passage 58 falls ot until it is no longer greaterthan that in low pressure accumulator 98. Valve 76 then opens andhydraulic uid flows from low pressure accumulator 98 to till theremainder of the cylinders 28. Thus, it will be seen, the rst portion ofthe compression stroke is activated by the high pressure fromcompensator 63 and the latter portion by the lower pressure fromaccumulator 98, the mean pressure being substantially greater than thatof accumulator 98 alone.

Resistance to turning a shaft 79 by an external load tends to raise thepressure in accumulator 50 and the maximum pressure in compensator 63,while lowering the pressure in accumulator 98. An increased load,therefore, produces an increased bounce pressure, and force, on pistons32 during the initial part of their inward, or compression, strokes anda reduced force during the balance of the compression stroke. Withsuitable proportioning of the relative lengths of the two portions ofthe strokes and the volumetric capacities of the accumulators 63 and 98,it is possible to obtain any desired bounce characteristics in responseto a wide variation in the external load. Thus, for example, a constantmean bounce effect may approximately be provided regardless of load,whereby uniformity of compression in the combustion space 13 is assuredregardless of the fuel charge injected through tube 38 at each stroke.Or, if desired, any variation from constant compression ratio may alsobe attained by changing the volumes of the accumulators 63 and 98 andthe area parameters of the pumping pistons 32 to imbalance the high andlow pressure portions of the bounce in either direction.

Control of fuel injection, and thus of the power irnpulse, may becoordinated with the torque demands on motor shaft 79 either manually orby an automatic regulator responsive to the pressure in some part of thesystem or to the pressure drop over the motor 78. Control systems ofthis type are known and need no additional description.

The operation of the engine is as follows: Fuel is injected throughinjector 38. The injector ymay be a manually or automatically controlledinjectorj its control being determined in response to the magnitude'ofexternal power demanded of the engine. Accordingly, the fuel controlcorresponds to the well known control of conventional diesel engines.When the engine is idling, the pistons reciprocate at low speed with thefuel injection being reduced to the point at which the above low speedis obtained. Fuel injection is increased in response to the increase inthe external load. The engine is stopped by shutting olf fuel, at whichtime the pistons return to their inner position due to the pressureexerted upon them by the uid in the auxiliary and the low pressureaccumulators; the pistons remain in their inner position as long as theengine is shut down. The engine is started by withdrawing the pistons totheir outward positions by means of an external mechanical starter,which may be similar to the automobile engine starter, connected to thepiston linkages, not shown in the drawing. Such starting means andlinkages are known to the art and need no illustration or description.The pistons are then released, the pressure in accumulator 98 drivingthem inward to produce a compression stroke and ignition of combustiblemixture of compressed air and dispersed fuel. Since the engine dependson the hydraulic bounce for producing compression stroke, it isnecessary to provide some auxiliary means for restoring the pressure inaccumulator 98 after long shut down periods. This means (notillustrated) may be a pressure-controlled pump, well known in the art.As mentioned previously, control of load and speed is obtained bymanually or automatically controlling the amount of fuel injected ateach stroke of the engine.

It has been stated in the introductory part of the specification thatthe maximum pressure in the auxiliary accumulator increases with theincrease in the external load. It follows that the compensating actionof the auxiliary accumulator during the compression stroke increaseswith the increase in the external load. This is the type of relationshipwhich is desired since the pressure in the low pressure accumulatordecreases with the increase in load and such decrease must becompensated for by the inverse increase in Apressure obtainable in theauxiliary accumulator.

What is claimed as new is:

l. In combination with a two stroke free piston engine having a mutuallyactuating free power piston and pumpmg piston, a hydraulic motor, andworking uid for ope'rating said motor, a closed hydraulic powertransmission system for said working fluid connecting said engine andsaid motor, comprising a high pressure accumulator connected to thepumping piston to receive fluid under pressure therefrom during thepower stroke of said e11- gine and to the input of said motor to deliverfluid under pressure thereto, a Alow pressure accumulator connected tothe output of the motor to receive uid therefrom and to said pumpingpiston to return fluid thereto during the compression stroke of saidengine, means preventingthe ow of uid from said engine to said lowpressure accumulator during the power stroke of said engine, and anauxiliary accumulator connected directly to said pumping piston toreceive uid under pressure during the power stroke of said engine andreturn the same without external regulation to said pumping piston forthe compression stroke of said engine.

2Q In combination with a two stroke free piston engine having a mutuallyactuating free power piston and pumping piston, a hydraulic motor, andworking fluid for operating said motor, a closed hydraulic powertransmission system for said working tluid connecting said engine andsaid motor, comprising a high pressure conduit connecting said pumpingpiston to the input of said motor to deliver said uid during the powerstroke of saidvengine, a low pressure conduit connecting the output ofthe motor to said pumpingpiston to return said fluid during thecompression stroke of said engine, valve means for preventing the owA ofuid in said low pressure conduit from said engine during the powerstroke thereof, a high pressure accumulator interposed in said highpressure conduit, a low pressure accumuator interposed in said lowpressure conduit andan axuiliary ac cumulator, interposed directlybetween said high and low pressure accumulators and said pumping piston,to receive fluid under pressure during the power stroke of said engineand return the same without external regulation to said piston for thecompression stroke of said engine.

3. In combination with a two stroke free piston engine having a mutuallyactuating free power piston and pump- 6 ing piston, a hydraulic motor,and working fluid for operating said hydraulic motor, a closed hydraulicpower transmission system for said working uid connecting said engineand said motor, comprising a high pressure accumulator, a 'conduithydraulically connecting said accumulator to said pumping piston and theinput of said motor, valve means in said conduit permitting the ow ofsaid lluid from said pumping piston to said accumulator only during thepower stroke of said engine, a low pressure accumulator. a low pressureconduit hydraulically connecting said low pressure accumulator to saidpumping piston and the output of said motor, valve means in said lowpressure conduit preventing the ow of said Huid from said pumping pistonto said low pressure accumulator during the power stroke of said engine,and an auxiliary high pressure accumulator having a direct hydraulicconnection with said pumping piston to receive uid under pressure duringthe power stroke of said engine and return the same, without externalregulation, together with fluid from said low pressure accumulator toproduce pressure on said pumping piston for the compression stroke ofsaid engine.

4. The system according to claim 3 wherein said auxiliary accumulator ispre-proportioned with respect to said high and low pressure accumulatorsto accommodate variations in output of said engine.

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